New York: Julian Assange fuelled conspiracy theories by falsely suggesting that a murdered Democratic party employee leaked damaging information about Hillary Clinton's campaign to WikiLeaks rather than Russian hackers, according to special counsel Robert Mueller's report.

A veteran Democratic Party consultant said Mueller's report proved once and for all that Assange is "a monster, not a journalist" and that this should not be forgotten following his recent arrest in London.

In July 2016 WikiLeaks published approximately 20,000 emails that had been stolen from the Democratic National Committee (DNC) and later released a massive cache of emails that had been sent or received by Clinton's campaign manager John Podesta.

In the days following Rich's death, right-wing conspiracy theories began circulating that he had been assassinated and that his murder was connected to the DNC email hack.

WikiLeaks founder Julian Assange in 2016. Credit:AP

The claim has been debunked by multiple fact checking sites and the Metropolitan Police Department of the District of Columbia said Rich's murder was the result of a bungled attempted robbery.

"Beginning in the summer of 2016, Assange and WikiLeaks made a number of statements about Seth Rich, a former DNC staff member who was killed in July 2016," Mueller's report states in a section on Russian hacking.

"The statements about Rich implied falsely that he had been the source of the stolen DNC emails.

"On August 9, 2016, the @WikiLeaks Twitter account posted: 'ANNOUNCE: WikiLeaks has decided to issue a US$20k reward for information leading to conviction for the murder ofDNC staffer Seth Rich.'

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"Likewise, on August 25, 2016, Assange was asked in an interview, 'Why are you so interested in Seth Rich’s killer?' and responded, 'We’re very interested in anything that might be a threat to alleged Wikileaks sources.'"

Later in the interview Assange said: "If there’s someone who’s potentially connected to our publication, and that person has been murdered in suspicious circumstances, it doesn’t necessarily mean that the two are connected.

"But it is a very serious matter...that type of allegation is very serious, as it’s taken very seriously by us."

Mueller's report shows that Assange went far further than WikiLeaks' usual practice of not revealing its sources. Instead he actively spread misinformation about the genesis of the Clinton leaks.

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Even after the US intelligence community publicly stated that Russia was behind the hacking operation, Assange continued to deny that Russian hackers were behind the leaks.

He told Dana Rohrabacher, a former pro-Putin Republican congressman, that the hack was an "inside job" and claimed to have evidence that Russia was not behind the hacks.

Brad Bauman, a progressive political consultant and former spokesperson for the Rich family, said: "Assange did untold damage to a grieving family in order to try and hide his work with Russian intelligence to destabilise American democracy.

"In the process he misled hundreds of thousands of Americans who are just trying to make sense of our country in difficult and complicated times.

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"Yesterday’s report proved he is a monster, not a journalist, and I hope that’s not lost in the ongoing debate around his recent arrest."

Rich's brother Aaron said: "I hope that the people who pushed, fuelled, spread, ran headlines, articles, interviews, talk and opinion shows, or in any way used my family’s tragedy to advance their political agendas - despite our pleas that what they were saying was not based on any facts - will take responsibility for the unimaginable pain they have caused us."

In his report Mueller also quotes private messages by Assange explaining his preference that Donald Trump to win the 2016 election rather than Hillary Clinton.

Assange described Clinton as "a bright, well connected, sadisitic [sic] sociopath" in a November 2015 message.

He also said Clinton would have "greater freedom" to start a war than a Republican president.

Having been kicked out of the Ecuadorian embassy in London, Assange faces jail time for breaching UK bail laws. He may also be extradited to the US over allegations he conspired with former US military analyst Chelsea Manning to download classified material about the Iraq War.